0020-6814/07/950/798-13 $25.00 798
Early Orogenic History of the Eastern Himalayas:
Compositional Studies of Paleogene Sandstones from Assam,
Northeast India
ASHRAF UDDIN,1 PRANAV KUMAR,
Himalayan Research Laboratory, Department of Geology and Geography, Auburn University, Auburn, Alabama 36849 AND J. N. SARMA
Department of Applied Geology, Dibrugarh University, Dibrugarh 786004, Assam, India
Abstract
Thick Eocene–Oligocene sequences, exposed near the Margherita-Changlang area, northeast Assam represent detritus derived from the early Himalayan and Indo-Burman orogenic belts, extending the 18–0 Ma record recovered from drilling the distal Bengal Fan. Sandstones from the Eocene Disang Group (Qt68F3L29; total quartz–feldspar–lithic fragments) and the lower Oligocene Naogaon Formation (Qt69F6L25) are compositionally and texturally immature, composed mainly of quartz, sedimentary and low-grade- metamorphic lithic fragments (including abundant chert), and plagioclase. Sandstones of the overlying middle and upper Oligocene Baragolai (Qt66F12L22) and Tikak Parbat (Qt82F4L14) formations are similar but also contain significant amounts of volcanic and higher grade metamorphic detritus. These sandstones are clearly derived from an orogenic source, exposing and eroding sedimentary and low-grade metamorphic units to form the older sandstones, followed by increasing contributions from volcanic and higher grade metamorphic rocks during dep-osition of the middle and upper Oligocene sandstones. In contrast, Eo-Oligocene strata (Eocene: Qt99F1L0; Oligocene: Qt90F3L7) from the neighboring Bengal Basin contain angular quartzose sands that represent first-cycle detritus, most likely from the Indian craton. The Bengal Basin was pro-tected from orogenic sedimentation during the Eocene–Oligocene, either by a barrier to sediment transport (a peripheral forebulge or a marine basin) or by distance, prior to the approach of the basin toward Asia. Motion of this part of the Indian plate relative to now-adjacent Southeast Asia was most likely accomplished along strike-slip faults, like the N-S–trending Kaladan fault, located just east of the Bengal Basin. Similarity in modal composition (quartzolithic to phyllarenitic) of Paleogene sequences of Assam and basins south of the Himalayan western syntaxis suggests that the Hima-layan emergence was not strongly diachronous, with initial collision and uplift at both syntaxial areas occurred in the Eocene.
Introduction
THE COLLISION OF India with Eurasia provides a spectacular lesson in plate tectonics. Timing of the collision near the eastern syntaxis is very poorly known (Packham, l996; Rowley, l996), however, and improved resolution on the timing would aid in developing more accurate models for deformation in the eastern Himalayas. Data bearing on the timing of collision come mainly from areas west of the cen-tral Himalayas. Although most workers suggest that India began to collide with Eurasia at around 50 Ma, others propose an earlier collision at about 70 Ma
(Yin and Harrison, 2000). Even less well understood is the location of the boundary between India and Indochina through time. Most workers place the main boundary between India and Indochina for the past 13 million years along the Sagaing fault in Myanmar (formerly Burma; Mitchell, 1993; Fig. 1). Total displacement on the Sagaing fault is not well known, but evidence on offset of ophiolitic rocks, and on opening of the Andaman Sea suggest about 400 to 500 km of right slip (Curray, 1989). NUVEL-1A plate reconstructions place Assam, the northeast corner of India, some 3000 ± 250 km south of Eurasia at about 50 Ma, and more recent recon-structions decrease this by only a few hundred kilo-meters (Gordon et al., 1999).
Evidence of the early collision in the eastern Hima-layas should be recorded in the stratigraphic record of basins south of the mountain belt. Paleogene strata of the deep-sea Bengal fan have not yet been recovered (only back to about 18 or 17 Ma; Cochran, 1990). Paleogene sandstones of the onshore delta of the Bengal Basin are quartzose, suggesting deriva-tion most possibly from non-orogenic sources (Uddin and Lundberg, 1998a). More proximal to the eastern Himalayas is the Assam Basin of India, a foreland basin with over 6 km of Eocene to Pleis-tocene marine to terrestrial strata deposited on continental crust. Thus it is anticipated that the initiation of collision may be recorded by these predominantly non-marine or deltaic strata, in that collision likely began in the submarine realm. How-ever, considering the modern Taiwan collision, it appears that a sizeable mountain belt can emerge in
a relatively short time span (within 1 m.y.; Dorsey, 1988). In the case of Taiwan, shallow-marine to non-marine sediments were deposited on continental crust of the downgoing plate within 1 m.y. or so from the inception of collision (Covey, 1986). Further-more, it is important to note that the Assam sequence records the very early collision, because the initial detritus is rich in sedimentary lithic frag-ments and it subsequently shifted to dominance by meta-sedimentary lithic fragments.
This study reports modal analyses of Eocene and Oligocene sandstones exposed near the Margherita-Changlang area of northeast Assam, India. Compo-sitional data were collected to constrain the prove-nance of these deposits, and to compare them with coeval sequences elsewhere in the foreland; this should help decipher the early erosional record of the eastern Himalayas in order to further elucidate FIG. 1. Map of South Asia showing lithotectonic belts of Himalayan and Indo-Burman orogens and locations of Assam and the Bengal Basin, along with other reference locations mentioned in the text. The Indian shield and Shillong Plateau expose Precambrian crystalline rocks. Approximate limits of the Indus and Bengal fan are shown. Deep Sea Drilling Project sites 217, 218, 222, 223, and 224 and area drilled by Ocean Drilling Program Leg 116 are shown in the Bengal and Indus fans. Framed area is shown in detail in Figure 2 (after Uddin and Lundberg, 1998a).
the history of collision between northeast India and Asia.
Regional Geologic Setting
The Assam Basin is bounded by the Indian cra-ton and the Shillong Plateau, a Precambrian massif, to the west; by the eastern segment of the Himalayas to the north; the Mishmi Hills in the northeast; the Indo-Burman Ranges to the east and immediate south; and the Bengal Basin of Bangladesh and the Bengal deep-sea fan to the southwest (Fig. 1). The eastern Himalayan syntaxis is located only about 150 km NNE of Assam; parts of the Himalayas and the Indo-Burman Ranges are located even closer (Fig. 2). The northernmost extension of the Indo-Burman Ranges merges with the E-W–trending Himalayas at the Eastern Himalayan syntaxis. The
Himalayas consist of six longitudinal lithotectonic units juxtaposed along generally N-dipping thrust faults (Le Fort, 1996). From north to south (Fig. 1), these are the: (1) Trans-Himalayas, consisting of calc-alkaline plutons; (2) Indus suture zone, expos-ing ophiolitic bands representexpos-ing the zone of colli-sion between India and Eurasia; (3) Tibetan Himalayas, represented by fossiliferous Cambrian to Eocene sediments; (4) Higher Himalayas, located north of the Main Central Thrust, composed of schists, gneisses, and leucogranites; (5) Lower or Lesser Himalayas, composed of unfossiliferous Pre-cambrian and Palaeozoic sedimentary rocks, and crystalline rocks; and (6) Sub-Himalayas, represent-ing Miocene to Pleistocene molasse-type deposits of the Siwaliks. The N-S–trending Indo-Burman Ranges east and south of the Assam-Bengal system consist of early Tertiary synorogenic sediments, FIG. 2. Map showing the Assam and Bengal basins and their tectonic elements such as the eastern Himalayas and Indo-Burman Ranges. Areas enclosed by the Naga and Disang thrusts form the Schuppen belt. Samples for this study were collected from the northeastern part of the Schuppen belt (Margherita-Changlang) of Assam. The Shillong Plateau, Mikir Hills, and Mishmi Hills are uplifted blocks of Precambrian massifs. The Dauki fault demarcates the Shillong Plateau from the Sylhet trough of the Bengal Basin. The Kaladan fault, located east of the Chittagong Hills of the Bengal Basin, separates the Assam sequences from the Bengal Basin (after Hutchison, 1989).
schists, and ophiolitic belts (Fig. 1; Brunnschweiler, 1966; Sengupta et al., 1990). Crystalline rocks, predominantly gneisses of Precambrian age, make up the bulk of the Indian craton that is sporadically overlain by Permian Gondwana deposits and Creta-ceous flood basalts of the Rajmahal Traps (Hutchi-son, 1989). Crustal material of a pre-Gondwana landmass crops out in the Mikir Hills, the Shillong Plateau, and the Mishmi Hills, most of which lie outside Assam. The Shillong Plateau, which is a major geomorphic feature in the region, was uplifted to its present height in the Pliocene (Johnson and Nur Alam, 1991). The southern edge of the plateau is bounded by the Dauki fault (Fig. 2; Uddin and Lundberg, 2004).
Several thrust faults bound the Margherita-Changlang area of northeast Assam, including the Naga thrust to the northwest and Disang thrust to the southeast (Fig. 2). This thrust-bounded area is also called the “Schuppen belt” (Rangarao, 1983). The Naga thrust is a major décollement in the study area. Thrusting began in the late Eocene or early Oligocene and continued into the late Pliocene; total shortening is estimated to be about 300 km (Evans, 1964; Saikia, 1999). The imbricate belt of the Naga thrust developed through compression during sub-duction (Fig. 2; Saikia, 1999). Geomorphically, the Assam and Bengal basins are separated by the Mikir Hills, the Shillong Plateau, and the Schuppen belt. Thick successions of Cenozoic basin fill have been drilled and exposed in the Sylhet trough of the Bengal Basin and uplifted along the Chittagong fold belts of the eastern Bengal Basin (Fig. 2). The Chit-tagong fold belts comprise tight NNW-trending folds along the eastern edge of the foredeep. The Kohima-Patkai synclinorium is developed in the southern and southeastern parts of the Schuppen belt, and extends to the folded belt of the Sylhet trough and Chittagong Hills (Fig. 2; Dasgupta, 1984). These fold belts represent a series of N-S–trending anti-clinal ridges and synanti-clinal valleys, an arcuate belt that is convex toward the west. The fold belt shows an increase in structural complexity toward the east, into the Arakan Yoma–Chin Hills and the Indo-Burman Ranges (Fig. 2). The latter are bounded by two N-S–trending right lateral faults, Sagaing to the east and Kaladan to the west, adjacent to the Bengal Basin (Uddin and Lundberg, 2004). Although the Sagaing fault is commonly recognized as a right-lateral fault in Southeast Asia (e.g., Curray, 1989; Mitchell, 1993; Uddin and Lundberg, 1998a), the Kaladan fault is not that popularly known. Although
this has a thrust component (Sikder, 1998), designa-tion of the Kaladan fault as a right-lateral one has been promoted by Murphy (1988) and Zutshi (1993). The Kaladan fault trends NE-SW along the Kaladan River, between the eastern boundary of Bangladesh and western Myanmar (Fig. 2; Murphy, 1988; Zutshi, 1993; Sikdar, 1998). This fault is traceable on satellite images from the Disang thrust on the north to offshore Myanmar on the south, a distance of few hundred kilometers.
Assam Paleogene Sequences
The stratigraphic framework of Assam is based mainly on biostratigraphy, predominantly using palynology, with correlations depending on litho-stratigraphy (Evans, 1964; Sinha and Sastri, 1973; Rangarao, 1983). The basin sequences have also been correlated by seismic stratigraphy by various industry groups, including the Oil and Natural Gas Commission of India (Saikia, 1999).
The Paleogene section of the Margherita-Chang-lang area used in this study (Table 1) comprises the upper Eocene Disang Group (up to 3 km thick), the lower Oligocene Naogaon Formation (up to 2.2 km), the middle Oligocene Baragolai Formation (up to 3.3 km), and the upper Oligocene Tikak Parbat Formation (~0.7 km; Table 1). The Oligocene forma-tions make up the Barail Group. The thickness of each unit decreases generally to the west (Rangarao, 1983).
The Disang Group is marine, based on marine fossils, radiolarian cherts, and other typical deep-marine deposits. The Disang sequence consists of fissile, carbonaceous mudrocks with fine-grained sandstone. Nagappa (1959) reported arenaceous for-aminifera from the top part of Disang and suggested a late Eocene age. Evans (1964) found Nummulites from sandy shale of Disang and suggested a late Eocene age. The upper part of the Disang represents an argillaceous facies analogous to the Eocene Sylhet and Kopili formations (shelf equivalents in the Upper Assam Plains and Mikir hills; Rangarao, 1983).
The Naogaon Formation consists mostly of fine-grained sandstones with subordinate siltstones, claystones, and shales, showing flaser and lenticular bedding. The middle unit of the Baragolai Forma-tion is dominantly argillaceous with thin siltstones and sandstones. Shales in this unit are dark grey and commonly show concretions. The Tikak Parbat For-mation is composed dominantly of grey, moderately
sorted sandstones; minor siltstones and thick coal beds are also present in this unit. These Oligocene units have been interpreted as brackish-water and deltaic deposits (Rangarao, 1983).
Methods
Twenty-three representative Eocene–Oligocene sandstone samples from Assam were selected for modal analysis on the basis of appropriate grain size and low alteration. Most of the samples are highly indurated. A few unconsolidated sand samples chosen were sieved, and the fractions coarser than 0.063 mm were epoxied into plugs for thin-section preparation. Petrographic analyses were conducted following the Gazzi-Dickinson method, counting sand-sized minerals included in lithic fragments as the mineral phases rather than the host lithic frag-ment (i.e., Ingersoll et al., 1984). All thin sections were stained for plagioclase and potassium feldspar, following techniques modified from Houghton (1980). At least 300 framework points were counted per sample, with 400 framework points counted for samples with greater compositional diversity. Selected thin sections were also counted a second time by a different person in order to evaluate oper-ator error.
Point-counting parameters and recalculated parameters are defined in Table 2. Normalized modal data are given in Table 3 and representative photomicrographs are shown in Figure 3. Polygons surrounding mean values are calculated as sample standard deviations, although these do not represent true standard deviations for constrained-sum data (see Ingersoll et al., 1984); they are shown to
indi-cate the variability of values for each group. Ternary diagrams using major detrital components, mono-crystalline grains, and the phaneritic lithic frag-ments were constructed in order to visualize variations in sand composition and to help interpret the tectonic provenance (i.e., Dickinson, 1985). Normalized modal data are depicted graphically in Figure 4A and 4B.
Assam Paleogene Sandstone Compositions
Modal analytical data from Eocene–Oligocene sequences in Assam are summarized below for the various stratigraphic units, from oldest to youngest. Disang Group
Sandstones from the Eocene Disang Group (Qt68F3L29;Figs. 3A, 4A, and 4B) are composed of fine- to medium-grained, subangular to angular grains, containing mostly monocrystalline quartz, and also foliated and equant polycrystalline quartz, plagioclase, sedimentary and metamorphic lithic fragments of phyllite grade and fine-grained quartz-mica-chlorite schist. Sedimentary and low-grade metasedimentary lithic fragments suggest derivation of sediments from proximal orogenic sources. Like the plagioclase, the large angular monocrystalline quartz could have been derived from a volcanic source, or possibly from a granitic source, although thealmost complete lack of alkali feldspar suggests otherwise.
Naogaon Formation
Sandstones from the lower Oligocene Naogaon Formation (Qt69F6L25; Figs. 3B, 4A, and 4B) are TABLE 1. Paleogene Stratigraphy of the Margherita-Changlang Area in Upper Assam
Chronostratigraphy Group Formation Thickness (m) Brief lithology
Oligocene Barail Tikak Parbat 500 to 700 Sandstones, thin-bedded grey sandy siltstone Baragolai 2700 to 3300 Predominantly shale with subordinate thin
sandstone beds and prominent coal seams Naogaon 1040 to 2200 Thinly bedded sandstone, thin subordinate
shale
Late Eocene Disang Disang 2000 to 3300 Fine-grained sandstone with subordinate dark-gray shale rich in carbonaceous mat-ter and massive siltstone with concretions
quartzolithic and contain subangular to angular grains of monocrystalline and polycrystalline quartz, mostly plagioclase feldspar, and sedimentary and metamorphic lithic fragments. Lithic fragments in this unit are more diverse compared to the Eocene Disang Group sandstones.
Baragolai Formation
Sandstones from the middle Oligocene Baragolai Formation (Qt66F12L22; Figs. 3C, 4A, and 4B) com-prise mono-and polycrystalline (also sheared) quartz, feldspar (mostly plagioclase, with chlorite and epidote inclusions), sedimentary lithic fragments of shale, argillite and siltstone, and metamorphic lithic frag-ments of phyllite grade, fine- to medium-grained quartzose-mica schists, and chlorite-quartz-epidote-zoisite schists. Chert grains are abundant (Fig. 3C). Volcanic lithic fragments are also present, mostly of mafic lithologies with lathwork and local microlitic textures. Some of these volcanic lithic fragments of Baragolai sandstones show massive alteration to chlo-rite and possible epidote, probably representing a mild metamorphic overprint, although some alteration during burial diagenesis may also have occurred.
Lower and middle Oligocene sandstones also suggest a proximal orogenic source because the detritus is composed of sedimentary, metasedimentary, volcanic, and metavolcanic lithologies. Sheared quartz grains apparently were derived from zones of deformation. As with the older Disang unit, the lack or near absence of alkali feldspar suggests no significant granitic source rocks for the lower Oligocene Naogaon Formation. Lath-shaped plagioclase grains probably represent volcanic phenocrysts. Chlorite-quartz-epidote (zoisite) schists may have been derived from low-grade meta-morphism of calcareous shales or mafic volcanic rocks. Rare grains of amphibole also suggest a medium-grade metamorphic source.
Tikak Parbat Formation
Sandstones from the upper Oligocene Tikak Parbat Formation (Qt82F4L14; Figs. 3D, 4A, and 4B) are texturally immature, with angular to subangular fragments, and are coarser than the older units. These upper Paleogene sandstones are also compo-sitionally immature, consisting primarily of grains containing monocrystalline quartz showing undulose extinction. Sheared quartz, quartz-mica schist, TABLE 2. Recalculated Modal Parameters of Sand and Sandstones Used in This Study
Quartzose grains (Qt = Qm + Qp), where Qt = total quartzose grains
Qm = monocrystalline quartzose grains (> 0.625 mm) Qp = polycrystalline quartz grains, including chert grains Feldspar grains (F = P + K)
F = total feldspar grains P = plagioclase feldspar grains K = potassium feldspar grains
Unstable lithic fragments (L = Ls + Lv + Lm; L = Lsm + Lvm; Lt = Ls + Lv + Lm + Qp) L = total aphanitic lithic fragments
Lt = total aphanitic lithic fragments, including polycrystalline quartz and chert Ls = sedimentary lithic fragments, mostly argillites
Lv = volcanic lithic fragments
Lm = very low to intermediate grade metamorphic lithic fragments Lsm = sedimentary and metasedimentary lithic fragments Lvm = volcanic, hypabyssal, metavolcanic lithic fragments
TABLE 3. Normalized Modal Analyses of Paleogene Sandstones from Assam, India
QtFL (%) QmFLt (%) QmPK (%) QpLvmLsm (%) LsLvLm (%)
Sample number Qt F L Qm F Lt Qm P K Qp Lsm Lvm Ls Lv Lm
Tikak Parbat Fm. (Upper Oligocene)
T-17 95 1 4 59 1 40 99 1 0 90 8.9 0.6849 43 7 50 T-12 78 8 13 42 8 49 83 11 6 73 27 0 62 0 38 T-10 83 4 14 49 4 47 93 4 3 71 29 0 63 0 37 T-7 77 3 19 58 3 39 94 3 3 50 50 0 56 0 44 T-1 74 5 21 33 5 62 86 14 0 66 31 3.0973 58 9 33 Mean (n = 5) 82 4 14 48 4 47 91 7 2 70 29 0.7565 56 3 40 Standard deviation 8.4 3 7 11 3 9 6.3 6 2 14 15 1.3418 8 5 6.8
Baragolai Fm. (Middle Oligocene)
B-25 70 17 13 37 17 46 68 8 23 71 29 0 53 0 47 B-21 64 8 28 48 8 44 85 12 3 37 63 0 26 0 74 B-19 62 17 21 49 17 34 74 24 2 38 53 8.6207 38 14 49 B-14 64 11 25 54 11 35 85 15 0 30 64 5.9322 45 8 47 B-7 72 6 22 54 6 39 89 11 0 45 40 14.754 28 27 45 B-4 67 10 23 54 10 36 85 15 0 37 55 8 39 13 48 Mean (n = 6) 66 12 22 49 12 39 81 14 5 43 51 6.2178 38 10 51 Standard deviation 3.8 5 5 6.6 5 5 8.1 5 9 15 14 5.6431 10 10 11
Naogaon Fm. (Lower Oligocene)
NB-2 73 3 24 66 3 31 96 4 0 22 77 1 15 1 83 NA-7 76 3 22 70 3 28 96 4 0 22 78 0 37 0 63 NB-6 72 10 18 52 10 39 84 10 5 54 46 0 58 0 42 NA-5 71 1 28 23 11 76 97 3 0 63 37 0 14 0 86 NA-2 52 13 35 47 13 40 78 3 18 12 84 4.6875 72 5 23 Mean (n = 5) 69 6 25 52 6 43 90 5 5 34 64 1.1375 39 1 60 Standard deviation 9.6 5 7 19 5 20 8.6 3 8 22 21 2.0312 25 2 27
Disang Group (Eocene)
D-23 74 4 22 51 4 45 93 7 0 51 49 0 68 0 32 D-22 64 3 34 50 3 48 95 5 0 30 70 0 73 0 27 D-16 78 1 22 61 1 39 99 1 0 44 56 0 89 0 11 D-15 67 2 30 56 2 42 96 4 0 27 69 4.0698 61 6 34 D-12 67 4 30 59 4 37 94 6 0 21 79 0 56 0 44 D-8 69 4 27 57 4 39 93 7 0 30 70 0 88 0 12 D-2 65 3 32 54 3 43 95 5 0 26 74 0 48 0 52 Mean (n = 7) 68 3 29 56 3 41 95 5 0 33 67 0.5814 69 1 30 Standard deviation 4.9 1 4 3.9 1 4 2 2 0 11 10 1.5382 16 2 15 Paleogene Mean (n = 23) 71 6 23 51 6 43 89 8 3 45 53 2.1733 51 4 45
chlorite-mica schist, black shale, and polycrystal-line quartz (equant and foliated) are also present. These sandstones also contain abundant stretched quartz grains, chert, epidote, muscovite, and biotite. Feldspars are sparse, and actinolite and epidote schists are also rare. The presence of quartz-mica schists, chlorite-mica schist, and the abundance of detrital mica in the upper Oligocene to Neogene sandstones suggest a low- to intermediate-grade metamorphic source for the sandstones.
Interpretation of Assam Paleogene Sandstone Modes
All Paleogene (Eocene and Oligocene) units analyzed plot in the “recycled orogenic” provenance fields of QtFL and QmFLt diagrams (Fig. 4A;
Dick-inson, 1985). These sandstones are quartzolithic (Table 3; Q71F6L23) and phyllarenitic, and contain more sedimentary and metasedimentary lithic frag-ments (Ls51Lv4Lm45). In the monocrystalline QmPK diagram, most of the samples plot near the Qm pole (Fig. 4A). Volcanic components are generally scarce in Assam sandstones, with a peak in abundance in the middle Oligocene Baragolai Formation that has higher feldspar contents (Figs. 4A and 4B). Sample T-17, which has a very quartzose composition, was collected from strata that are probably transitional between the Oligocene Tikak Parbat Formation and the Neogene Surma Group (Table 3). For reference, the Paleogene sandstones from the Bengal Basin are also plotted in Figure 4 (Eocene—Be; Oligocene— Bo). The sandstones from the Bengal Basin show FIG. 3. Representative photomicrographs of sandstones from Assam, India. A. Eocene Disang Group: framework grains are dominantly quartz (Qm), with sedimentary lithic fragments (Ls), plagioclase feldpars (plag), and chert grains. B. Lower Oligocene Naogaon Formation: monocrystalline quartz grains (Qm), sedimentary lithic fragments (Ls), plagio-clase feldspar (plag), and chert grains. C. Middle Oligocene Baragolai Formation: monocrystalline (Qm) and polycrys-talline quartz grains (Qp), potassium feldspar (K-spar), and chert grains. D. Upper Oligocene Tikak Parbat Formation: monocrystalline quartz grains (Qm), sedimentary (Ls) and metamorphic (Lm) lithic fragments, and mica. All these framework grains suggest orogenic derivation. In contrast, the Oligocene Barail Formation from the Bengal Basin shows subangular quartz grains with rare or no feldspar and lithic fragments (Fig. 6A; Uddin and Lundberg, 1998a).
FIG. 4. A. Ternary diagrams showing sandstone modes of Paleogene sandstones from Assam (QtFL, QmFLt, QmPK; see Table 1 for definitions). Data plots show means (indicated by numbers 1–4 and standard deviation polygons for each stratigraphic unit. Provenance fields are from Dickinson (1985). For comparison, distribution of the Paleogene Bengal Basin samples (Be = Bengal Basin Eocene; Bo = Bengal Basin Oligocene) is also shown in all the diagrams (from Uddin and Lundberg, 1998a). Note that the detrital modes of Paleogene sandstones from Assam plot in a “recycled orgenic” field that is different from the the Paleogene sandstones of the Bengal Basin. Although standard deviations are not strictly valid statistically for constant-sum, constrained compositional data, polygons are shown to indicate ranges of values. B. Ternary diagrams showing lithic and polycrystalline modes of Paleogene sandstones from Assam (LsLvLm, QpLvmLsm; see Table 1 for definitions). Data plots show means (indicated by numbers 1–4 and standard deviation polygons for each stratigraphic unit. These plots do not show presence of much volcanic lithic fragments except the mid-Oligocene Baragolai unit. For comparison, distribution of the Paleogene Bengal Basin samples (Be = Bengal Basin Eocene; Bo = Bengal Basin Oligocene) is also shown in all the diagrams (from Uddin and Lundberg, 1998a). All these plots show dominance of sedimentary and metamorphic lithic fragments in the Paleogene sequences of Assam. Volcanic lithic fragments are not that significant, except in the mid-Oligocene Baragolai unit.
more maturity (placed close to the quartz poles than the Paleogene sandstones of Assam.
Paleogene Sandstones across the Himalayan Foreland
Paleogene sandstones from Assam are composi-tionally quite different from coeval sandstones of the adjacent deltaic Bengal Basin, but similar to coeval sandstones of the foreland basins south of the west-ern Himalayas. Eocene–Oligocene sandstone(s) from the Bengal Basin are less indurated and are dominantly quartzose (Qt90F3L7 to Qt99F1L0). Many of the quartz grains are coarse and most are suban-gular to ansuban-gular (Fig. 6A of Uddin and Lundberg, 1998a). Almost all are monocrystalline grains, with very minor polycrystalline grains, and sedimentary lithic fragments, scarce metamorphic lithic frag-ments, and no identifiable volcanic detritus. All of the rare feldspar grains are potassium feldspars. These quartz arenites are interpreted to have been derived from the adjacent Indian craton (Uddin and Lundberg, 1998a). The abundance of quartz and scarcity of both feldspar grains and lithic fragments in Bengal Basin sandstones also suggest a possible source terrane with low relief, with intense chemical weathering due to the position of the basin close to the equator during the Paleogene. Given sufficiently intense chemical weathering, the possibility also exists that these quartzose sandstones were derived from an orogenic source (Uddin and Lundberg, 1998a). In more close proximity toward the north-west of the Bengal Basin and north-west of Assam, but still in the eastern half of the Himalayas, in the western and central Nepal, the Paleocene fluvial to shallow-marine Amile Formation and Eocene shallow-marine to shal-low-marine Bhainskati Formation are pure quartz-arenites (Fig. 1; DeCelles et al., 1988). The lower Miocene nonmarine Dumri Formation in western Nepal is quartzolithic (Qt72F4L24; DeCelles et al., 1998) with very little feldspar, most of which is plagioclase. Zircon dates from these units suggest a possible Himalayan source (DeCelles et al., 1998). In the western Himalayan basins, the upper Paleocene to lower Miocene synorogenic sediments that began to fill the evolving foreland basins that developed ahead of the southward-advancing Hima-layas comprise terrestrial sediments of the lithofelds-pathic Chulung La Formation (Fig. 1; Paleocene to Oligocene; Qt24F26L50), quartzolithic tidal-flat to fluviatile deposits of the Murree Supergroup (Paleo-cene to Oligo(Paleo-cene; Qt68F5L27; Garzanti et al., 1987;
Critelli and Garzanti, 1994), the quartzolithic shal-low-marine Subathu Formation (upper Paleocene to middle Eocene; Qt63F7L30; Najman and Garzanti, 2000), and tidal flat to alluvial quartzolithic Dagshai Formation (upper Oligocene; Qt58F1L31; Najman and Garzanti, 2000; Fig. 1). Like the sandstones from Assam and unlike the sandstones from the adjacent Bengal Basin, these units contain abun-dant metasedimentary, volcanic, and sedimentary lithic fragments and ophiolitic detritus, beginning early in the Paleogene (Garzanti et al., 1987; Critelli and Garzanti, 1994).
Early Orogenic History of the Eastern Himalayas
The Eocene–Oligocene sandstones from Assam were clearly derived from an orogenic source, exposing and eroding sedimentary and low-grade metamorphic units to form the older sandstones, followed by increasing contributions from volcanic and higher grade metamorphic rocks during deposi-tion of the middle and upper Oligocene sandstones (Fig. 5A). The Assam sandstones provide clear evidence that orogeny had begun in the eastern Himalayas by the Eocene, in contrast to the early Miocene initiation suggested by the apparently first-cycle Paleogene quartz arenites (Uddin and Lund-berg, 1998a, 1998b) and subsurface lithofacies patterns of Miocene (Uddin and Lundberg, 1999) of the Bengal Basin. The more proximal Assam sequence apparently records the early stages of orogenic activity; whereas the initial detritus is rich in sedimentary lithic fragments, later sandstones show a subsequent shift to dominance by metasedi-mentary lithic fragments.
Heavy-mineral contents in Oligocene sequences from Assam are composed mostly of zircon, tourma-line, and rutile (ZTR) that are also associated among others with chloritoid, epidote, garnet, hornblende, kyanite, staurolite, and spinel, suggesting an oro-genic source (Uddin et al., 2007). Microprobe study of garnets and chrome-spinel grains from Paleogene sequences of Assam also suggest a Himalayan source material (or ophiolites) and/or the Indo-Burmese ophiolitic belts (Kumar and Uddin, 2004). Presence of dominantly ZTR minerals among the nonopaque variety in the Eocene and Oligocene sequences of the Bengal Basin suggests intense post-depositional weathering and does not obviously suggest an orogenic source (Uddin and Lundberg, 1998b). Heavy-mineral assemblages in both the
Assam and Bengal basins become more diverse in Miocene and younger formations, indicating deriva-tion from orogenic belts (Uddin et al., 2007).
The Bengal Basin may have been protected from orogenic sedimentation during Eocene and Oligo-cene time, either by a barrier to sediment transport (a peripheral forebulge, or a marine basin, for exam-ple) or simply by distance (Fig. 5A). Early uplifts of the Indo-Burman Ranges could potentially have
acted as a barrier; however, that seems unlikely because the westward-encroaching ranges were probably located farther east relative to the Bengal Basin during the Paleogene than in the Miocene (Mitchell, 1993; Uddin and Lundberg, 1999). These compositional data also suggest that the Assam and Bengal basins were latitudinally farther apart prior to early Miocene time, and as a consequence, were receiving detritus from two distinct sources. The two sequences are presently exposed on either side of the N-S–trending right-lateral Kaladan fault (Murphy, 1988; Zutshi, 1993), between the eastern fold belts of the Bengal Basin and western folds in Assam, India (Figs. 1 and 2). This transpressional fault seems to be resulting from oblique conver-gence of India with Indochina. India has been moving both north and eastward; the northerly motion has been attributed to the Miocene opening of the Andaman Sea resulting in N-directed move-ment of India along right-lateral faults (Pivnik et al., 1998). A strong candidate for such a fault is the Kaladan fault (Uddin et al., 2007). These two dis-tinct sequences were in close proximity by early Miocene time because both are covered by lower Miocene strata (the Bhuban Formation of the lower Surma Group; Johnson and Nur Alam, 1991; Uddin and Lundberg, 2004) that are similar in provenance (Fig. 5B; Godwin et al., 2001; Uddin et al., 2007).
More regionally, the thick Eocene–Oligocene sands from the Assam sequences are similar in com-position to those of the western Himalayan foreland, suggesting that the initial collision of Northeast and Northwest India with Asia was not strongly diachro-nous. This non-diachronous convergence is also supported by isotopic and compositional studies (DeCelles et al., 1998) and paleomagnetic study (Patzelt et al., 1996) and by work on subsequent Miocene metamorphism and cooling history of the two syntaxial areas (Nanga Parbat in the west and Namche Barwa in the east; Ding et al., 2001). This assumes, however, that Assam was initially part of Indochina.
The active Kaladan fault appears to (geographi-cally) separate the two Eocene–Oligocene sequences in Assam and the Bengal Basin. Detritus in the latter was apparently derived from the neigh-boring Indian craton, accumulating on crust of the Indian plate prior to arrival of the clastic wedge shed from the approaching orogeny. If true, then the Miocene strata represent an overlap assemblage, signifying the “docking” of this part of the Indian plate with proximal terranes of Asia. One possible FIG. 5. Schematic paleogeographic reconstruction of the
Himalayan and surrounding areas during the Paleogene time showing tectonic elements of Assam, India, and Bengal Basin in (A) pre-Miocene and (B) Miocene time. The Bengal Basin may have been transported close to Assam during the Miocene along right-lateral faults (i.e., the Kaladan fault) located east of the basin.
explanation of the contrast in sediment source is that the part of the Indian plate represented by the Bengal Basin was still far to the south of Asia until the Miocene, when it arrived close enough to receive detritus from the orogenic highlands fringing Asia’s southern boundary. It is possible that the sequences preserved in the Bengal Basin and Assam were orig-inally deposited on two separate lithospheric plates, with Assam as part of Indochina (Fig. 5A).
Conclusions
Paleogene sandstone composition from the study area of northeastern Assam indicates recycled orogenic derivation. The Assam sandstones differ from coeval sandstones in the adjacent Bengal Basin, which are texturally immature first-cycle quartz arenites that were most likely derived from the neighboring Indian craton. The Bengal Basin was probably protected from orogenic sedimentation during the Paleogene, either by a barrier to sedi-ment transport or distance. If “distance” was the cause, then the part of the Indian continent repre-sented by the Bengal Basin was far to the south of Asia until the early Miocene. Motion of this part of the Indian plate relative to Southeast Asia (Indo-china) was most likely accomplished along right-lateral faults, like the N-S–trending Kaladan fault, located east of Bangladesh.
If the analyzed Paleogene sequences of Assam were deposited on Indian continental crust, then the Himalayan collision was not strongly diachronous, with initial collision of both Northeast and North-west India in the Eocene forming the two syntaxial bends of the Himalayas. This suggestion of non-diachroneity is also supported by similarity in composition and thickness of Paleogene strata in basins (Assam, India and Pakistan) near the two Himalayan syntaxes.
Acknowledgments
Thanks to Suvrat Kher for confirmational point counts. Neil Lundberg, Clark Burchfiel, Roy Odom, and Suvrat Kher helped with various aspects of the work, including editing parts of the manuscript. Sev-eral students from Dibrugarh University (Assam) helped during field work and sample collection for the project. Mr. T. Bordoloi helped with logistics in Digboi, Assam. Reviews by Peter DeCelles and Ray Ingersoll have significantly improved the manu-script. M. Shamsudduha and Khandaker Zahid
helped draft some figures. PK received a grant-in-aid support from Geological Society of America. This manuscript is supported by U.S. National Science Foundation grant EAR-0310306 awarded to AU.
REFERENCES
Brunnschweiler, R. O., 1966, On the geology of the Indo-Burman ranges (Arakan coast and Yoma, Chin Hills, Naga Hills): Geological Society of Australia Bulletin, v. 13, p. 137–194.
Cochran, J. R., 1990, Himalayan uplift, sea level, and the record of Bengal fan sedimentation at the ODP Leg 116 sites, in Cochran, J. R., Stow, D. A. V., et al., eds., Proceedings of the Ocean Drilling Program, Scientific Results, v. 116b: College Station, TX, Ocean Drilling Program, p. 397–414.
Covey, M., 1986, The evolution of foreland basins to steady state: evidence from the western Taiwan fore-land basin, in: Allen, P. A., and Homewood, P., eds., Foreland basins: International Association of Sedi-mentologists Special Publication no. 8, p. 77–90. Critelli, S., and Garzanti, E., 1994, Provenance of the
lower Tertiary Murree redbeds (Hazara-Kashmir syn-taxis, Pakistan) and initial rising of the Himalayas: Sedimentary Geology, v. 89, p. 265–284.
Curray, J. R., l989, The Sunda Arc: a model for oblique plate convergence, in: Proceedings of the Snellius II Symposium, theme: Geology and Geophysics of the Banda Arc and adjacent areas, Part 1. Netherlands Journal of Sea Research, v. 24, p. 131–140.
Dasgupta, S., 1984, Tectonic trends in Surma basin and possible genesis of folded belt: Geological Survey of India, Memoir, v. 113, p. 58–61.
DeCelles, P. G., Gehrels, G. E., Quade, J., and Ojha, T. P., 1998, Eocene–early Miocene foreland basin develop-ment and the history of Himalayan thrusting, western and central Nepal: Tectonics, v. 17, p. 741–765. Dickinson, W. R., 1985, Interpreting provenance relations
from detrital modes of sandstones, in Zuffa, G. G., ed., Reading provenance from arenites: Dordecht, The Netherlands, Riedel, p. 333–361.
Ding, L., Zhong, D., Yin, A., Kapp, P. and Harrison, T. M., 2001, Cenozoic structural and metamorphic evolution of the eastern Himalayan syntaxis (Namche Barwa): Earth and Planetary Science Letters, v. 192, p. 423– 438.
Dorsey, R. J., 1988, Provenance evolution and unroofing history of a modern arc-continent collision: Evidence from petrography of Plio-Pleistocene sandstones, east-ern Taiwan: Journal of Sedimentary Petrology, v. 58, p. 208–218.
Evans, P., 1964, The tectonic framework of Assam: Jour-nal of the Geological Society of India, v. 5, p. 80–96.
Garzanti, E., Baud, A., and Mascle, G., 1987, Sedimentary record of the collision with Eurasia (Ladakh Himalaya, India): Geodinimica Acta (Paris), v. 1, p. 297–312. Godwin, T., Uddin, A., and Sarma, J. N., 2001, Provenance
history of Neogene sandstones from the Assam basin, India [abs]: Geological Society of America Annual Proceedings, Abstracts with Programs, v. 33, p. 72. Gordon, R. G., Argus, D. F., and Heflin, M. B., 1999,
Revised estimate of angular velocity of India relative to Eurasia: EOS (Transactions of the American Geophysical Union), v. 80, p. F273.
Houghton, H. F., 1980, Refined techniques for staining plagioclase and alkali feldspar in thin section: Journal of Sedimentary Petrology, v. 50, p. 629–631. Hutchison, C. S., 1989, Geological evolution of Southeast
Asia: Oxford, UK, Oxford Science Publications, 368 p. Ingersoll, R. V., Bullard, T. F., Ford, R. L., Grimm, J. P., Pickle, J. D., and Sares, S. W., 1984, The effect of grain size on detrital modes: a test of the Gazzi-Dickin-son point-counting method: Journal of Sedimentary Petrology, v. 54, p. 103–116.
Johnson, S. Y., and Nur Alam, A. M., 1991, Sedimentation and tectonics of the Sylhet trough, Bangladesh: Geolog-ical Society of America Bulletin, v. 103, p. 1513–1527. Kumar, P., and Uddin, A., 2004, Chrome-spinel con-straints on provenance history of Cenozoic sediments from Assam, northeast India [abs]: Geological Society of America Annual Proceedings, Abstracts With Programs, v. 36, p. 505.
Le Fort, P., 1996, Evolution of the Himalaya, in Yin, A., and Harrison, M., eds., The tectonic evolution of Asia: New York, NY, Cambridge University Press, World and Regional Geology Series, p. 95–109.
Mitchell, A. H. G., 1993, Cretaceous–Cenozoic tectonic events in the western Myanmar (Burma)–Assam region: Journal of the Geological Society, London, v. 150, p. 1089–1102.
Murphy, R. W., 1988, Bangladesh enters the oil era: Oil and Gas Journal, February, p. 76–82.
Nagappa, Y., 1959, Foraminiferal biostratigraphy of the Creataceous: Eocene succession in the India-Pakistan-Burma region: Micropalaeontology, v. 5, p. 145–192. Najman, Y., and Garzanti, E., 2000, Reconstructing early
Himalayan tectonic evolution and paleogeography from Tertiary foreland basin sedimentary rocks, north-ern India: Geological Society of America Bulletin, v. 112, p. 435–449.
Packham G., l996, Cenozoic SE Asia: Reconstructing its aggregation and reorganization, in Hall, R., and Blundell, D., eds., The evolution of Southeast Asia: Geological Society (London) Special Publication No. 106, p. 123–153.
Patzelt, A., Li, H., Wang, J., and Appel, E., 1996, Palaeo-magnetism of Cretaceous to Tertiary sediments from southern Tibet: Evidence for the extent of the northern margin of India prior to the collision with Eurasia: Tectonophysics, v. 259, p. 259–284.
Pivnik, D. A., Nahm, J., Tucker, R. S., Smith, G. O., Nyein, K., Nyunt, M., and Maung, P. H., 1998, Polyphase deformation in a fore-arc/back-arc basin, Salin Sub-basin, Myanmar (Burma): American Association of Petroleum Geologists Bulletin, v. 82, p. 1837–1856. Rangarao, A., 1983, Geology and hydrocarbon potential of
a part of Assam-Arakan basin and its adjacent area: Petroleum Asia Journal, v. 6, p. 127–158.
Rowley, D. B., l996, The age of initiation of collision between India and Asia: A review of stratigraphic data: Earth and Planetary Science Letters, v. 145, p. 1–13. Saikia, M. M., 1999, Indo-Burman orogenic belt: its plate
tectonic evolution, in Verma, P. K., ed., Geological studies in the eastern Himalayas: Delhi, India, Pilgrims Book (Pvt) Ltd., p. 19–39.
Sengupta, S., Ray, K. K., Acharyya, S. K., and De Smith, J. B., 1990, Nature of ophiolite occurrences along the eastern margin of the Indian plate and their tectonic significance: Geology, v. 18, p. 439–442.
Sikdar, A.M., 1998, Tectonic evolution of eastern folded belt of Bengal Basin: Unpubl. Ph.D. thesis, Dhaka University, Dhaka, Bangladesh, 175 p.
Sinha, R. N., and Sastri, V. V., 1973, Correlation of the Tertiary geosynclinal sediments of the Surma valley, Assam and Tripura State (India): Sedimentary Geology, v. 10, p. 107–134.
Uddin, A., Kumar, P., Sarma, J. N., and Akhter, S. H., 2007, Heavy-mineral constraints on provenance of Cenozoic sediments from the foreland basins of Assam, India and Bangladesh: Erosional history of the eastern Himalayas and the Indo-Burman ranges, in Mange, M. A., and Wright, D. T., eds., Heavy minerals in use: Amsterdam, The Netherlands, Elsevier, Devel-opments in Sedimentology, v. 58, in press.
Uddin, A., and Lundberg, N., 1998a,Cenozoic history of the Himalayan-Bengal system: Sand composition in the Bengal basin, Bangladesh: Geological Society of America Bulletin, v. 110, p. 497–511.
Uddin, A., and Lundberg, N., 1998b, Unroofing history of the Eastern Himalaya and the Indo-Burman ranges: Heavy-mineral study of Cenozoic sediments from the Bengal basin, Bangladesh: Journal of Sedimentary Research, v. 68, p. 465–472.
Uddin, A., and Lundberg, N., 1999, A paleo-Brahmapu-tra? Subsurface lithofacies analysis of Miocene deltaic sediments in the Himalayan-Bengal system, Bang-ladesh: Sedimentary Geology, v. 123, p. 227–242. Uddin, A. and Lundberg, N., 2004, Miocene
sedimenta-tion and subsidence during continent-continent colli-sion, Bengal Basin, Bangladesh: Sedimentary Geology, v. 164, p. 131–146.
Yin, A., and Harrison, M. T., 2000, Geologic evolution of the Himalayan-Tibetan orogen: Annual Review of Earth and Planetary Science, v. 28, p. 211–280. Zutshi, P. L., 1993, Tectonics and hydrocarbon prospects of
Cachar-Tripura, eastern region, India: Bulletin of Oil and Natural Gas Corporation Ltd., v. 30, p. 97–123.
